Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 240 Section 21 - VLANs and VTP - Review of Collision vs. Broadcast Domains In a previous guide, it was explained that a “collision domain” is a segment where a collision can occur, and that a Layer-2 switch running in Full Duplex breaks up collision domains. Thus, Layer-2 switches create more collision domains, which results in fewer collisions. However, Layer-2 switches do not break up broadcast domains, and thus belong to only one broadcast domain. Layer-2 switches will forward a broadcast or multicast out every port, excluding the port the broadcast or multicast originated from. Only Layer-3 devices can break apart broadcast domains. Because of this, Layer-2 switches are not well suited for large, scalable networks. Layer-2 switches make forwarding decisions solely based on Data-Link layer MAC addresses, and thus have no way of differentiating between one network and another. Virtual LANs (VLANs) Virtual LANs (or VLANs) separate a Layer-2 switch into multiple broadcast domains. Each VLAN is its own individual broadcast domain (i.e. IP subnet). Individual ports or groups of ports can be assigned to a specific VLAN. Only ports belonging to the same VLAN can freely communicate; ports assigned to separate VLANs require a router to communicate. Broadcasts from one VLAN will never be sent out ports belonging to another VLAN. Please note: a Layer-2 switch that supports VLANs is not necessarily a Layer-3 switch. A Layer-3 switch, in addition to supporting VLANs, must also be capable of routing, and caching IP traffic flows. Layer-3 switches allow IP packets to be switched as opposed to routed, which reduces latency. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 241 VLAN Example Consider the following example: Four computers are connected to a Layer-2 switch that supports VLANs. Computers A and B belong to VLAN 1, and Computers C and D belong to VLAN 2. Because Computers A and B belong to the same VLAN, they belong to the same IP subnet and broadcast domain. They will be able to communicate without the need of a router. Computers C and D likewise belong to the same VLAN and IP subnet. They also can communicate without a router. However, Computers A and B will not be able to communicate with Computers C and D, as they belong to separate VLANs, and thus separate IP subnets. Broadcasts from VLAN 1 will never go out ports configured for VLAN 2. A router will be necessary for both VLANs to communicate. Most Catalyst multi-layer switches have integrated or modular routing processors. Otherwise, an external router is required for inter-VLAN communication. By default on Cisco Catalyst switches, all interfaces belong to VLAN 1. VLAN 1 is considered the Management VLAN (by default). * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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